It's just the way ark works with deciding which things (walls, ceilings, ramps, etc) to attach to, and - yes - it's annoying. They added the 'Q' key, a while back to help, some, though. I was able to get mine to line up by using 'Q' to rotate through the placements.

(The REAL annoyance is when you use 'Q' to rotate through the placements and the system STILL tries to help you and places it in a different spot. They need to get RID of the placement assitance and just let us use the 'Q' and 'E' keys for placement locations/rotations.

I can't get mine to place without a gap even *when* I'm using 'q' to control the placement. I love the mod, but I wince every time I walk throuhg a room where there are lines of sunlight on the fllor because the roofing tiles won't cover the whole room.

Normaly this should not happen if you build with CKF Parts ONLY.
However it does happen if you mix CKF with vanilla or S+ parts (e.g. foundations).
Is is a known behaviour but is not considered a bug as ExileAcid stated in this thread: http://psteamcommunity.yuanyoumao.com/workshop/filedetails/discussion/764755314/1326718197227958468/